The present invention relates to a method for producing an optical information recording disk such as a read-only optical disk, a write-once type optical disk, a read/write optical disk, or the like.
In an optical disk such as a video disk or the like, the radial interval between rows of pits or guide grooves concentrically or spirally formed on a recording surface of the disk, namely, the track pitch, is set at a minute value of about 1.6 .mu.m. When such an optical disk is mounted on a turntable of a reproducing apparatus, the spindle of the turntable is fitted into a center hole of the optical disk. Accordingly, the positioning of the optical disk relative to the spindle is extremely important with respect to the accuracy in rotation of the optical disk. Further, although a tracking servo is provided, if the displacement of the center of the optical disk is large, tracking by the laser beam pickup may be impossible because the range of the tracking servo is exceeded. Accordingly, when the center hole of an optical disk is formed, a highly accurate process is required so as to minimize the amount of displacement or eccentricity of the center point of the center hole from the center point of the recording surface constituted by the concentrically or spirally formed track.
Conventionally, formation of the center hole of an optical disk has been performed by cutting a substrate of a transparent circular plate prepared by injection molding of PMMA resin, PC resin, or the like. The center of the substrate (the center of the spiral of guide grooves) is determined using an optical instrument, and the substrate is bored while being firmly fixed. In the cutting process, however, there has been a problem in that the cutting machine is large in size and the substrate tends to suffer damage on its surface.
Alternatively, a metal mold having a protrusion corresponding to a center hole of an optical disk has sometimes been used so as to directly produce an optical disk having a center hole. In this case, however, there is a problem in that the accuracy is so low that a circular hole with acceptable circularity and concentricity cannot be obtained.
As shown in FIG. 1, there occurs a displacement or gap between a spindle 21 of a transferring apparatus and a substrate 22 in a transferring step of the conventional optical disk manufacturing process, which makes it impossible to correctly transfer the original recording pattern from the surface of a stamper 23 of the transferring apparatus onto the substrate 22 in such a manner as to make the center of the spiral track pattern coincide with the center of the substrate 22.